Many important commercial products, including optically functional products such as lenses, light fibers, optical screens and filters, reflective sheeting, and the like, have structures or are prepared from materials that are susceptible to physical damage. Alternatively, there may exist some other reason why such products benefit from protection against physical or mechanical damage. To protect these products, a tough, abrasion resistant "hardcoat" layer may be coated and cured onto their structures.
Abrasion resistant hardcoats can include a polymeric binder matrix formed from a curable material, and inorganic oxide materials suspended or dispersed within the polymeric binder. See, e.g., WO 96/36669 A1, which describes a hardcoat formed from a "ceramer" used, in one application, to protect the surfaces of retroreflective sheeting from abrasion. As is known in the art of ceramer compositions, ceramers can be derived from aqueous sols of inorganic colloids according to a process in which a curable binder precursor and other optional ingredients are blended into the aqueous sol. The resulting curable composition is dried to remove substantially all of the water. Solvent may then be added, if desired, in amounts effective to provide the composition with viscosity characteristics suitable for coating the composition onto a desired substrate. After coating, the composition can be dried to remove the solvent, and then exposed to a suitable source of energy to cure the binder precursor.
Optically functional products can include coatings having the primary function of enhancing or reducing light reflectance from the surface of a substrate. When such a coating reduces the amount of light reflected by the substrate, it is called "antireflective." When the coating enhances the amount of light reflected by the substrate it is called "reflective."
Antireflective (AR) coatings in particular are becoming increasingly important in commercial applications. The transparency of plastic or glass, in the form of doors, windows, lenses, filters, display devices (e.g., display panels) of electronic equipment, and the like, can be impaired by glare or reflection of light. To reduce the amount of glare on plastic or glass, the surface can include a layer of a metal oxide (such as silicon dioxide or indium tin oxide (ITO)), or suitably alternating layers of metal oxides, such as ITO/SiO.sub.2. For example, glass surfaces can typically have about 4% surface reflection, but with the aid of specialized coatings, such as multilayers of sputter deposited ITO/SiO.sub.2, surface reflection can be reduced to less than about 0.5% in the visible region of the spectrum (400-700 nm).
Importantly, the reflectivity or antireflectivity of a multi-layer optically functional composite article depends not only on the reflectivity of each layer, but also on the relative refractive indices of layers that are adjacent within the composite structure. Adjacent layers having similar or identical refractive indices will cause little or no additional reflection. But, if the indices of refraction of adjacent layers of a multilayer optically functional composite are different, this will cause reflectance of light at the interface of such adjacent layers, and diminish antireflective properties.
There is a need for chemical compositions that can function as abrasion-resistant "hardcoat" compositions. There exists an even more specific need for such hardcoats having optical properties (e.g., specific indices of refraction) wherein the hardcoat composition can be useful in optical product applications, for example in optically functional composites having reflective or antireflective properties.